Insulation-liquid-filled electrical equipment, such as oil-filled shunt reactors, bushings, and especially transformers such as power and distribution transformers, can be filled with insulation liquid, for example oil, for cooling and electrical insulation purposes. Faults inside the electrical equipment as well as degradation of the insulation liquid and of other insulation components such as insulation paper provided within the electrical equipment can form gasses which dissolve into the liquid.
It is important to detect such faults, errors and degradations, since transformers can be important for electrical power supply, and their failure can be very costly. Hence, a transformer is supposed to operate continuously and as error-free as possible over many years or even decades. For this reason, it is desirable that errors that can eventually cause failure of the transformer can be detected in time to take appropriate counter-measures.
Faults in insulation liquid-filled transformers can be generally accompanied by the development of gases dissolved in the insulation liquid and/or by an increase of moisture (e.g., water) in the insulation liquid. The quantity and composition of the decomposition gases and moisture is dependent on both the insulation liquid and the type and the amount of energy of the underlying defect. A large fault with high energy content, such as rapid overheating or arcing, would cause large amounts of gas to be produced in a short period of time, whereas the amount of gas produced by a small fault can be relatively smaller.
One reason for the formation of the gases is the decomposition of the insulation liquid or other parts, for example of cellulose or other solid insulators, caused by partial discharges and circulating currents, local overheating, high contact resistance or strong eddy currents and by arc discharges, or simply by aging. As a result, in a particular example, long-chain oil molecules of an insulation oil can be partially decomposed into gases. The hydrocarbon-based dielectric oil can produce, for example, free hydrogen, hydrocarbons (methane, acetylene, ethylene, ethane or the like), carbon dioxide and/or carbon monoxide as it thermally degrades or breaks down. Dielectric paper substrates can also decompose and produce, for example, carbon monoxide, carbon dioxide and/or water. Also, other gases such as oxygen can be produced. Additionally, moisture can also be produced by aging processes of the cellulose or exposure to ambient air. The moisture then diffuses into the insulation liquid. At higher concentrations, the moisture is deleterious for the insulation properties of the electrical equipment.
Therefore, an analysis of the insulation liquid gives an indication of the equipment's health status. For example, if the nature and amount of individual gases dissolved in the insulation liquid can be known, this information can be used to identify the type and severity of the corresponding electrical fault in the equipment. Even minute detected changes in the chemical composition of the gas produced and the rate of gas production over time can be important factors in determining the type of fault(s) involved, the evolution of the fault(s) and the potential consequences.
To verify the health status of the insulation liquid of such electrical equipment, two main methods are known: According to a first known method, also referred to as offline-method, samples of the insulation liquid can be regularly (for example, yearly) taken on-site and analyzed in a specialized laboratory by dissolved gas analysis. However, this offline-method is burdensome and does not allow obtaining real-time data, and is of no further interest here even though it is the most widely used method.
According to a second known method, also referred to as online-method, measurements monitor the gas concentration in the insulation liquid directly and (quasi-)continuously. These on-line sensors range from high-end devices which specifically detect several gasses individually to more simple detectors. For example, commercial low-end devices can be configured to detect one gas or an unspecific combination of gases. However, if only one gas is detected, some faults in which this gas is not produced can be overlooked. On the other hand, if an unspecific combination of gases is detected, only little information about the nature of the fault is obtained, and also the risk of false alarms is increased.
Some known systems for detecting gases can be described in the following. RU 2,137,119 describes an ultrasonic detection of gases in electrically insulating oils.
U.S. Pat. No. 6,526,805 describes an apparatus that consists of a gas extraction cell, an infrcan bed gas analysis instrument, and a gas diaphragm pump, so that gas and water vapor components can be extracted from the insulating oil by permeating through a membrane in the gas extraction cell and can be then brought into the infrcan bed analysis instrument by the gas diaphragm pump. The infrcan bed analysis instrument performs a complex analysis of the extracted gas which reveals various information about the gas. However, such an instrument is fragile and must be calibrated periodically.
EP 1637879 describes an apparatus in which gases in transformer oil can be separated and passed to a measurement chamber in an analysis unit. Herein, two sensors can be used for a gas component (for example, hydrogen) in order to increase the accuracy and redundancy of the system. The sensors can be commercially available solid state sensors.
U.S. Pat. No. 3,866,460 describes an apparatus for detecting the presence of one or more specific gases in a fluid coolant. Therein, diffusion membranes separate the gases from the liquid coolant, and then separate hydrogen from the gas mixture. The hydrogen amount is then obtained from a pressure measurement of the hydrogen gas.
However, even though the known online systems allow much more detailed data to be obtained than the offline systems, some drawbacks and obstacles remain, such as complex sensor design, cross sensitivity among various gas components, problems due to sensor aging and drift, high cost, high maintenance requirements and/or limited life-time reliability of the sensors.